Currently, for a hemacytometer which performs volume metering with the use of a metering tube, both detecting the surface of a liquid with an optical sensor and detecting a floater with an optical sensor share the same principle that a change in light intensity caused by an object such as the liquid or the floater can be utilized to control a start or an end of the volume metering.
The principle for detecting a liquid surface with an optical sensor is described hereinafter. There are two optical sensors, a start sensor and an end sensor, provided respectively at the top and the bottom of a metering tube. When the liquid surface passes the start sensor at the top of the metering tube, the presence of the liquid surface causes a change in light intensity, and hence an electrical signal is generated resulting in the start of the volume metering/counting; when the liquid surface reaches the end sensor at the bottom of the metering tube, there is a change in light intensity at the end sensor, and hence another electrical signal is generated resulting in the end of the volume metering/counting. For this metering mode, there may be a problem of liquid side flow for a reagent in the metering tube due to the self-gravity of the liquid, that is, the liquid flows down quickly along a certain side of the inner wall of the metering tube, which causes the metering cannot be performed. In order to reduce the probability of side flow, a rinse with a relatively great surface tension has to be used in the metering tube for a cleanout and maintenance thereof. However, the side flow phenomenon may still occur when the metering tube dirties.
The principle for detecting a floater with an optical sensor is described hereinafter. When the floater passes the start sensor at the top of the metering tube, an electrical signal is generated resulting in the start of the volume metering/counting; when the floater reaches the end sensor at the bottom of the metering tube, another electrical signal is generated resulting in the end of the volume metering/counting. For this metering mode, although a relatively high metering precision may be achieved and the cleanout of the metering tube by the rinse may be avoided, the cost of the floater is rather high and the relevant cleanout is troublesome.